Speculation grows that Turnbull government MP John Alexander may resign

Voters in the federal seat of Bennelong could be forced to a pre-Christmas byelection amid mounting expectation that Liberal MP John Alexander will soon resign from Parliament over dual citizenship.

The embattled MP broke his silence on Friday to claim the UK Home Office has not responded to his request for clarification about his status - nearly five days after Fairfax Media revealed he was likely a UK citizen by descent.

When asked how much longer it would take, Mr Alexander replied: "Well that's the $64,000 question."

Asked directly if he would resign, Mr Alexander said he did not deal in "hypotheticals".

Mr Alexander would need to quit and trigger the byelection - which he plans to contest - no later than Monday to allow the minimum 33-day campaign required to hold the poll on December 16.

The weeks after that date - during the Christmas and New Year period,- would be impractical, pushing back the next likely date until at least early February.

A complication is that Mr Alexander would have to fully renounce his claim to UK citizenship before nominating as a candidate. If he failed to have written confirmation of his status before standing he could still be considered constitutionally ineligible.

The government is already fighting a byelection in Barnaby Joyce's seat of New England on December 2, with the prospect of more to come as the citizenship crisis rolls on.

Citizenship experts agree Mr Alexander is likely a British citizen by descent because his father, Gilbert Alexander, was born in England before moving to Australia as a young child.

The Bennelong MP had never renounced British citizenship and for days could not say for sure whether his father did so in the two year gap between the introduction of Australian citizenship in 1949 and his birth in 1951.

"My father, if he was still alive, would be 110," Mr Alexander told reporters on Friday. "He died thirty years ago, so the records are all very old and are in archives.

"When I have any knowledge, which is conclusive, I will make a statement. But I can say anything else until then."

While Mr Alexander could seek to delay the byelection by first seeking a referral to the High Court, as his case appears similar to those of Fiona Nash and Stephen Parry it would likely be pointless.

Prime Minister Malcolm Turnbull said on Friday that anyone who finds out they are a dual citizen should resign from Parliament.

"If you are satisfied that you are not constitutionally eligible to sit in the Parliament, you shouldn't be sitting. That is fundamental," he said.

Where there were "line ball" cases and people were not sure if they were ineligible they should be referred to the High Court, he told reporters in Vietnam.

Without Mr Alexander and Mr Joyce, the government would be down to a minority for the remaining two weeks of lower house sitting, which could lead to chaos on the floor as Labor seeks to test Mr Turnbull's control.

Under the British Nationality Act 1948, people born to British men were automatically given British citizenship by descent.

This is the law that conferred British citizenship on former senator Ms Nash and former One Nation senator Malcolm Roberts, who were kicked out of Parliament by the High Court last month. Former Senate president Stephen Parry resigned last week after revealing he was a dual British citizen through the same law.